Metal oxides and compounds are frequently reduced to lower oxidation states in order to obtain a desired intermediate oxide or compound or the elemental metal itself. For example, molybdenum trioxide (MoO.sub.3) can be reduced to molybdenum dioxide (MoO.sub.2) or other intermediate oxides or molybdenum metal (Mo) by heating MoO.sub.3 in the presence of a reducing gas.
U.S. Pat. No. 3,264,098 by Heytmeijer, issued Aug. 2, 1966, discloses a method for reducing molybdenum oxides to molybdenum in a fluidized bed. The reduction is accomplished in a stagewise manner using a reducing gas of a first temperature in a first stage and employing a reducing gas heated to a second temperature in a second stage. The reaction chamber must be vibrated in order to maintain the finely divided molybdenum compound in a fluidized state. One of the reasons given for stagewise reduction is to prevent the formation of coarse metal powder due to the presence of water vapor developed during the reaction.
U.S. Pat. No. 4,659,376 by Carpenter et al., issued Apr. 21, 1987, discloses the stagewise reduction of molybdenum oxide to molybdenum metal in a fluidized bed reactor. The process is said to reduce the content of impurities such as lead, zinc, bismuth and copper in the finished product. It is disclosed that mechanical stirring of the bed is required during the second stage. It is also disclosed that MoO.sub.3 will sublime at temperatures above about 650.degree. C., causing the bed to get sticky and eventually defluidize.
U.S. Pat. No. 2,398,114 by Rennie, issued Apr. 9, 1946, discloses a process for reducing granulated molybdenum trioxide to molybdenum dioxide and finally to molybdenum metal. In order to prevent the initial reduction of molybdenum trioxide to molybdenum dioxide from proceeding at too high a temperature, the reducing gas is diluted with a non-reducing gas. Examples of such diluting gases include steam and nitrogen. It is disclosed that a diluting gas is not necessary during the reduction of molybdenum dioxide to molybdenum metal. There is no disclosure of employing a fluidized bed reactor to accomplish the reduction.
U.S. Pat. No. 3,941,867 by Wilkomirsky et al., issued Mar. 2, 1976, discloses a process for oxidizing molybdenum disulfide (MoS.sub.2) to molybdenum trioxide in a fluidized bed. Refractory particles such as sand, alumina and magnesia are used to stabilize and improve the fluidization behavior of the bed and to prevent agglomeration and/or sintering of solids in the reactor. Additionally, it is disclosed that a scraping device such as rotary arm blades or a vibratory device can be employed to prevent build-up of material inside the reactor.
In typical prior art methods for reducing a molybdenum oxide in a fluidized bed reactor chamber, finely ground molybdenum oxide is fed into the chamber and a fluidizing gas is injected from the bottom to cause the molybdenum oxide to fluidize. As used herein, the term "molybdenum oxide" refers in general to the molybdenum compound introduced into the reactor chamber. It will thus be understood to refer to all molybdenum oxides such as, for example, molybdenum trioxide (MoO.sub.3), molybdenum dioxide (MoO.sub.2) and molybdenum sesquioxide (Mo.sub.2 O.sub.3). The chamber is heated and a reducing gas supplied. Because of the fluidized state of the molybdenum oxide particles, the reducing gas is able to surround each particle, thereby increasing the speed and completeness with which the reduction occurs.
Because molybdenum trioxide volatilizes at temperatures above approximately 650.degree. C. and would be lost to the reactor chamber, it is common to use a two-stage method for reduction of molybdenum trioxide to metal. In stage one, molybdenum trioxide is reduced at a temperature below about 600.degree. C. to produce molybdenum dioxide as follows: EQU MoO.sub.3 +H.sub.2 .fwdarw.MoO.sub.2 +H.sub.2 O+heat.
In stage two, molybdenum dioxide begins to reduce to molybdenum at approximately 760.degree. C. as follows: EQU MoO.sub.2 +2H.sub.2 +heat.fwdarw.Mo+2H.sub.2 O.
Reduced product is removed from the reactor chamber through an overflow discharge tube. If the product is molybdenum dioxide, the process is repeated a second time at a higher temperature and the final product, molybdenum metal, is discharged through the overflow tube.
Because the reducing gas must contact each molybdenum oxide particle in order for complete reduction to occur, it is important that the particles in the chamber remain in a fluidized state. This is particularly critical during the second stage reduction process in which the molybdenum metal particles tend to stick together (agglomerate) causing defluidization, thus stopping the process.
Past efforts to eliminate the problem of defluidization have included the use of mechanical stirrers within the reactor chamber and external vibratory devices to prevent particles from sticking together or to separate the particles which have stuck together. It would be advantageous to provide a nonmechanical means for maintaining or restoring bed fluidization in a fluidized bed reactor chamber.